Light source with multi-longitudinal mode continuous wave output based on multi-mode resonant OPO technology

ABSTRACT

A light source providing multi-longitudinal resonant waves, particularly by utilizing an optical parametric oscillator (OPO) to produce a broadband emission spectrum. By configuring the system to pump the OPO far above the oscillation threshold, tunable light of macroscopic power with a short coherence length is provided. The coherence may be further shortened by additional longitudinal mode scrambling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Application Ser. No.62/561,428, filed Sep. 21, 2017, entitled “LIGHT SOURCE WITHMULTI-LONGITUDINAL MODE CONTINUOUS WAVE OUTPUT BASED ON MULTI-MODERESONANT OPO TECHNOLOGY,” which is incorporated by reference herein inits entirety.

FIELD OF THE INVENTION

The present invention generally relates to a light source providingmulti-longitudinal resonant waves, and more particularly, is related toa light source which utilizes an optical parametric oscillator (OPO) toproduce a broadband emission spectrum.

BACKGROUND

An optical parametric oscillator (OPO) is a light source emittingradiation with properties comparable to that of a laser. OPOs arenonlinear devices that split short wavelength pump photons into twolonger wavelength photons, namely signal and idler photons. Thewavelengths of the signal and idler photons are not independent fromeach other, but may be tuned in wavelength.

As shown by FIG. 1, an OPO converts an input laser wave (the “pump”)with frequency ω_(p) into two output waves of lower frequency (ω_(s),ω_(i)) via second-order nonlinear optical interaction. The sum of thefrequencies of the output waves is equal to the input wave frequency:ω_(s)+ω_(i)=ω_(p). For historic reasons, the output wave with the higherfrequency ω_(s) is called the signal, and the output wave with the lowerfrequency ω₁ is called the idler. Because the OPO does not convert allthe input energy into the signal and idler, a residual pump wave is alsooutput.

OPOs need an optical resonator, but in contrast to lasers, OPOs arebased on direct frequency conversion in a nonlinear crystal rather thanfrom stimulated emission. OPOs exhibit a power threshold for an inputlight source (pump), below which there is negligible output power in thesignal and idler bands.

OPOs include an optical resonator (cavity) and a nonlinear opticalcrystal. The optical cavity is an arrangement of mirrors that forms aresonator for light waves. Light confined in the cavity is reflectedmultiple times resulting in a multi-pass through the nonlinear crystal.The optical cavity serves to resonate at least one of the signal andidler waves. In the nonlinear optical crystal, the pump, signal andidler beams overlap.

While conventional lasers produce limited fixed wavelengths, OPOs may bedesirable because the signal and idler wavelengths, which are determinedby the conservation of energy and momentum (via phase matching), can bevaried in wide ranges. Thus it is possible to access wavelengths, forexample in the mid-infrared, far-infrared or terahertz spectral region,which may be difficult to obtain from a laser. In addition, OPOs allowfor wide wavelength tunability, for example, by changing thephase-matching condition. This makes OPOs a useful tool, for example,for laser spectroscopy.

In addition, while prior light sources such as spectrally filteredplasma sources and supercontinuum whitelight lasers are available, suchlight sources suffer from poor photon (energy) efficiency (typically afew mW output power per nm). On the other hand, OPO/non-linear optics(NLO) technology may offer significantly higher energy efficiency withmore narrow band output powers of greater than 50 mW. Thus, whilesupercontinuum and plasma sources produce a broad spectrum from which(for many applications requiring narrower linewidths) parts are cut off,OPOs are capable of producing a tunable comparatively narrow band output(so no waste of power by filtering out). Therefore, there is a need inthe industry to address one or more of the above mentioned shortcomings.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a continuous wave (cw)light source based on OPO technology, plus potential further NLOprocesses. In particular, while typical cw OPOs have a singlelongitudinal mode resonant wave, the present invention is configured toprovide high power tunable wavelength broadband emissions (shortcoherence). According to embodiments of the present invention, an OPO ispumped far above threshold to enable a multi-mode operation of theresonant wave, potentially also generating Raman-lines. In particular,the present invention provides a multi-longitudinal mode (greater than 3modes) resonant OPO with potentially controllable mode content plusoptional longitudinal mode scrambling. The high resonant wave powerproduced enables further efficient intracavity NLO mixing (e.g., SHG,second harmonic generation) to generate multimode and/or broadened atwavelengths shorter than that of the resonant wave.

According to one aspect, the present invention provides a light sourceincluding a pump source configured to produce a pump beam, and an OPOhaving an optical cavity containing a crystal configured to receivelight from the pump source and produce a first output light beam and asecond output light beam, the OPO having an OPO-oscillation thresholdlevel. In particular, the pump source is configured to produce the pumpbeam at a power sufficiently above the OPO-oscillation threshold suchthat the OPO produces broadband and/or multi-mode emission.

Embodiments according to this aspect may include one or more of thefollowing features. The pump source may be configured to produce thepump beam at a power exceeding about 3 times the OPO-oscillationthreshold. The pump source may be configured to produce the pump beam ata power exceeding about 5 times the OPO-oscillation threshold. The lightsource may further include a longitudinal mode-scrambling mechanismdisposed on the resonant OPO-wave. The longitudinal mode-scramblingmechanism may be a mechanical scrambling mechanism. The longitudinalmode-scrambling mechanism may be selected from one or more of a ditheredoptical resonator length, an intra-cavity etalon, a refractive gratingand a non-linear crystal, wherein the longitudinal mode-scramblingmechanism is configured to modulate a phase-matching condition and/orwavelength-dependent gain or losses of the OPO. The longitudinalmode-scrambling mechanism may be selected from one or more of anintra-cavity electro-optic phase modulator, an intra-cavityelectro-optic etalon, or a non-linear crystal, and the longitudinalmode-scrambling mechanism may be configured to electro-opticallymodulate the nonlinear phase-matching condition and/orwavelength-dependent gain or losses of the OPO. The light-source mayfurther include a second nonlinear crystal within the optical resonator,wherein the second crystal is configured for additional SHG, SFG or OPOprocesses. The second crystal may be configured to enable the lightsource to access additional wavelength ranges with broadband and/ormultimode emission having a bandwidth greater than 300 GHz. A secondpump-source may be configured to pump the second nonlinear crystal.

According to another aspect, the present invention provides a method ofproducing a broadband and/or multi-mode emission light source utilizingan Optical Parametric Oscillator (OPO), including: providing a pumpsource configured to produce a pump beam; providing an OpticalParametric Oscillator (OPO) including an optical cavity containing acrystal configured to receive light from the pump source and produce afirst output light beam and a second output light beam, the OPO havingan OPO-oscillation threshold level; and producing a pump beam with thepump source, the pump beam having a power at least about 3 times theOPO-oscillation threshold.

Embodiments according to this aspect may include one or more of thefollowing features. The OPO produces a broadband and/or multi-modeemission, and the method further includes longitudinal mode-scramblingon a resonant OPO-wave. The longitudinal mode-scrambling may be donemechanically. The longitudinal mode-scrambling may be carried out bydithering an optical resonator length of the OPO, providing an OPOintra-cavity etalon, providing a refractive grating, or providing anon-linear crystal, wherein the longitudinal mode-scrambling modulatesthe phase-matching condition and/or wavelength-dependent gain or lossesof the OPO. The longitudinal mode-scrambling may be carried out by usingan intra-cavity electro-optic phase modulator, an intra-cavityelectro-optic etalon, or a non-linear crystal, wherein the longitudinalmode-scrambling modulates a phase-matching condition and/orwavelength-dependent gain or losses of the OPO electro-optically. Themethod may further include providing a second nonlinear crystal withinthe optical resonator, wherein the second crystal is configured toperform additional SHG, SFG or OPO processes. The second crystal may beconfigured to enable the light source to access additional wavelengthranges with broadband and/or multimode emission having a bandwidthgreater than 300 GHz. The method may further include providing a secondpump source and pumping the second nonlinear crystal with the secondpump source.

The present light systems beneficially provide light generation whichfinds use in a variety of applications including microscopy and biotech.

Other systems, methods and features of the present invention will be orbecome apparent to one having ordinary skill in the art upon examiningthe following drawings and detailed description. It is intended that allsuch additional systems, methods, and features be included in thisdescription, be within the scope of the present invention and protectedby the accompanying claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprincipals of the invention. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. In the drawings,each like component is referenced by a like numeral. For purposes ofclarity, every component may not be labeled in every drawing. In thedrawings:

FIG. 1 is a general schematic diagram of a prior art OPO.

FIG. 2 is a schematic diagram of an OPO pumped by a high power pumpsource according to a first embodiment of the present invention.

FIG. 3 is a schematic diagram of an OPO pumped by a high power pumpsource according to a second embodiment of the present invention.

FIG. 4 is a schematic diagram of an OPO pumped by a high power pumpsource according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions are useful for interpreting terms applied tofeatures of the embodiments disclosed herein, and are meant only todefine elements within the disclosure. No limitations on terms usedwithin the claims are intended, or should be derived, thereby. Termsused within the appended claims should only be limited by theircustomary meaning within the applicable arts.

Normally a light-source having more than 1 longitudinal mode is called“multi-longitudinal mode”. However, as used within this disclosure,“multi-longitudinal mode” refers to a number of longitudinal modesgreater than 3. A number of modes of 2 or 3 may be called “fewsingle-modes” in this context.

As used within this disclosure, “broadband” refers to a bandwidthgreater than 300 GHz, no matter if formed by multiple modes, singlebroadened line(s) or a spectral distribution, and no matter if before orafter longitudinal mode scrambling.

As used within this disclosure, “longitudinal mode scrambling” refers toa method for fast frequency-tuning of modes, continuously or viahopping. In this context, reference to “fast” frequency tuning of modesmeans faster than can be timely resolved for an application. One exampleof a fast frequency tuning of modes is a repetition rate of greater than100 Hz (however, this is merely an example, and it is to be understoodthat reference to fast frequency tuning of modes in the presentinvention is not limited only to values of greater than 100 Hz).

As used within this disclosure, OPO generally refers to a continuouswave OPO (cw-OPO), rather than a pulsed OPO. In general, “continuouswave” or “CW” refers to a laser that produces a continuous output beam,sometimes referred to as “free-running,” as opposed to a q-switched,gain-switched or mode locked laser, which has a pulsed output beam.

As used within this disclosure, “mirror” refers to an optical elementhaving at least one reflective surface. The reflective surface mayreflect light received from one direction, but transmit light receivedfrom other directions. The reflective surface may reflect somewavelengths and transmit other wavelengths. Further the reflectivesurface may partially transmit and partially reflect some wavelengths.

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

In general, embodiments of the present invention include devices andmethods for producing broadband laser radiation, particularly by using amulti longitudinal resonant mode OPO. This is in contrast with priorOPOs, which are specifically configured so as to be resonant on a singleor substantially single mode. The present embodiments achieve thedesired broadband laser radiation by pumping the OPO far above theoscillation threshold. Such broadband linewidth or multi-mode lightsources are generally known to have short coherence length.

Referring now to the various figures of the drawing wherein likereference characters refer to like parts, there is shown in FIG. 2 oneembodiment of the light source in accordance with the present invention.As shown, a high power pump source 110 pumps the OPO 150, which in turnproduces an emission spectrum. According to an exemplary embodiment, thehigh power pump source 110 may be in the form of a common high-powerlaser or an amplified diode or laser. The light source of the presentembodiment is configured such that the whole emission spectrum of theOPO 150 is spectrally broadened and multimode. This may be accomplishedby forcing the OPO to resonate on multi-modes and/or by broadening modesby pumping the OPO 150 far above the threshold to allow multi-mode OPOoscillation. As such, the high power pump source 110 may be one whichdelivers a power high enough to allow multi-longitudinal mode operationof the resonant OPO wave. In order to keep the resonant OPO wavesingle-longitudinal mode, a conventional OPO usually uses a maximumpump-power of approximately 2.5 times the OPO-oscillation threshold.According to embodiments of the present invention, in order to generatea multi-longitudinal resonant OPO wave, the pump-power preferablyexceeds about 3 times the OPO-oscillation threshold. According topreferred embodiments, the pump-power exceeds about 3.5 times, morepreferably about 4 times, more preferably about 5 times, and even morepreferably more than 5 times the OPO-oscillation threshold. As anon-limiting example, assuming an exemplary OPO oscillation threshold ofapproximately 2 W, then a high-power pump source delivering between 2and 5 Watt may keep the resonant wave being single-longitudinal mode,whereas for high-power pump levels greater than 5 Watts theresonant-wave may start being multi-longitudinal-mode and/or broadened.

As depicted in FIG. 2, the OPO 150 includes an optical resonator 152resonant for at least one of the waves generated inside a NLO-crystal154 configured for OPO-process. In order to increase the numbers andspacings of the modes, the embodiments may take into account abroadening of the gain curve. In particular, the NLO-crystal 154 may beprovided with a sufficient broad phase-matching bandwidth (e.g. realizedby using sufficiently short NLO-crystals) plus optionally a specialshaped gain-curve (e.g. by using a ferroelectrically poled chirped OPOcrystal or a ferroelectrically poled crystal with multi-grating alongthe beam path; alternatively or additionally temperature gradients alongthe direction of the beam-propagation may be applied to the crystal).The latter option may even enable control of a detailed shape of theemission spectrum. For example, in a crystal with chirped poling, thepoling period may slightly vary along the beam-propagation resulting ina broadened gain curve for an OPO. Using a crystal with multi-gratingalong the beam path produces a super-positioning of gain-curves relatedto the different poling sections. For example, one crystal may containseveral parallel chirped or multi-grating areas for widerwavelength-tuning. In some embodiments, a chirped-fanout grating may beused, having a gradual variation of poling period length along andperpendicular to the beam propagation direction.

If desired, additional scrambling means 158 for scrambling the resonantwave(s) may be provided. For example, such scrambling may be achieved byfast variation of the resonator-length or fast dithering on wavelengthselecting elements (e.g. etalon, diffraction-grating, etc.). One meansfor further structuring the OPO gain curve, for example, is the use of athick Etalon. Additional longitudinal mode scrambling may be taken intoaccount as well (e.g., fast dither of cavity length or effectiveEtalon-thickness or a diffraction-grating or effective ferroelectricpoling period lengths). A fast dither of the “cavity length” may be donemechanically, e.g. by mounting at least one of the cavity mirrors to apiezo-electric element; or it may be done by inserting an electro-opticphase modulator inside the resonator. A fast dither of the “effectiveEtalon thickness” may be done mechanically, for example by mounting anair-spaced etalon to a piezoelectric element or by mounting a solidetalon to a galvo which dithers its angle; or it may be done by makinguse of electro-optic properties of the material a solid etalon may bemade of. A fast dither of a “diffraction-grating” may be donemechanically, e.g. by mounting it to a galvo which dithers its angle.The “effective ferroelectric poling period length” may be ditheredmechanically by fast translation of a crystal with multi-poling periodsor a fan-out poling design or a poled crystal with temperature gradientalong the direction of translation; or it may be done by making use ofelectro-optic properties of the material of the nonlinear crystal; forboth types of methods the nonlinear phase-matching conditions may bemodulated. For optical materials having electro-optic properties, therefractive index n may be changed quickly by applying an electro-opticfield. The OPO 150 then outputs the light to an optional spectral filter160 which may be used to filter parts of a spectrum or change thespectral widths (for OPO-output).

FIG. 3 depicts another embodiment of the light source in accordance withthe present invention. This embodiment is similar to that set out inFIG. 2, with an additional NLO (non-linear-optical) crystal 156 toprovide further intracavity nonlinear frequency generation (for example,SFG=sum frequency generation or SHG=second harmonic generation)processes to access additional wavelength ranges. As discussed inconnection with FIG. 2, a high power pump source 110 (for example, acommon high-power near infrared (NIR) laser or an amplified diode orlaser) pumps the OPO 250 far above the threshold to produce an emissionspectrum that is spectrally broadened.

Similar to the embodiment described in connection with FIG. 2, the OPOmodule 250 of FIG. 3 includes an optical resonator 152 resonant for atleast one of the waves generated inside the NLO-crystal 154. The secondcrystal 156 may be configured for SHG- and/or SFG processes involvingthe resonant wave(s). The crystals 154, 156 may be provided with asufficiently large phasematching bandwidth, and may optionally have aspecial shaped Gain-curve (for example, by using a chirpedferroelectrically poled OPO crystal or a ferroelectrically poled crystalwith multi-grating along the beam path). If desired, additionalscrambling means 158 for scrambling the resonant wave(s) may beprovided. For example, such scrambling may be achieved by fast variationof the resonator-length or fast dithering on wavelength selectingelements. One means for further structuring the OPO gain curve, forexample, is the use of a thick Etalon. Additional longitudinal modescrambling may be taken into account as well (for example, fast ditherof cavity length or effective Etalon-thickness or a diffraction-gratingor effective ferroelectric poling period lengths). A fast dither of the“cavity length” may be done mechanically, for example by mounting atleast one of the cavity mirrors to a piezo-electric element; or it maybe done by inserting an electro-optic phase modulator inside theresonator. A fast dither of the “effective Etalon thickness” may be donemechanically, for example by mounting an air-spaced etalon to apiezoelectric element or by mounting a solid etalon to a galvo whichdithers its angle; or it may be done by making use of electro-opticproperties of the material a solid etalon may be made of A fast ditherof a “diffraction-grating” may be done mechanically, for example bymounting it to a galvo which dithers its angle. The “effectiveferroelectric poling period length” may be dithered mechanically by fasttranslation of a crystal with multi-poling periods or a fan-out polingdesign or a poled crystal with temperature gradient along the directionof translation; or it may be done by making use of electro-opticproperties of the material the nonlinear crystal may be made of; forboth types of methods the nonlinear phase-matching conditions would bemodulated. For optical materials having electro-optic properties, therefractive index n may be changed quickly by applying an electro-opticfield. The OPO 250 then outputs the light to an optional spectral filter160 which may be used to filter parts of a spectrum or change thespectral widths (for OPO-/SFG-/SHG-outputs).

FIG. 4 depicts another embodiment of the light source in accordance withthe present invention. In contrast to FIG. 3, one SFG process mayinvolve a resonant OPO wave and a second light source 120. This SFGoutput may also have smaller coherence length since the resonant OPOwave is broadband. As discussed in connection with FIGS. 2 and 3, a highpower pump source 110 pumps the OPO 250 far above the threshold toproduce a broadband emission spectrum.

Similar to the embodiment described in connection with FIG. 3, the OPOmodule 250 of FIG. 4 includes an optical resonator 152 resonant at leastfor one of the waves generated inside the NLO-crystal 154. A secondcrystal 156 is configured for SHG- and/or SFG processes involving theresonant wave(s). A SFG-process may involve a resonant wave and thesecond light source 120. The crystals 154, 156 may be provided with asufficiently large phasematching bandwidth, and may optionally have aspecial shaped gain-curve (e.g. by using a chirped OPO crystal or acrystal with multi-grating along the beam path). If desired, additionalscrambling means 158 for scrambling the resonant wave(s) may beprovided. For example, such scrambling may be achieved by fast variationof the resonator-length or fast dithering on wavelength selectingelements. One means for further structuring the OPO gain curve, forexample, is the use of a thick Etalon. Additional longitudinal modescrambling may be taken into account as well (e.g., fast dither ofcavity length or effective Etalon-thickness or a diffraction-grating oreffective ferroelectric poling period length). The OPO 250 then outputsthe light to an optional spectral filter 160 which may be used to filterparts of a spectrum or change the spectral widths (forOPO-/SFG-/SHG-outputs).

As such, the present invention provides a light source which is capableof generating tunable light of macroscopic power with a short coherencelength. In particular, a pump-source with a high power level is utilizedto pump an OPO with a power far above OPO-oscillation threshold level.As a result, the OPO generates two new waves, generally referred to asthe signal and idler. The OPO cavity is resonant for at least one of thetwo new waves. According to the embodiments, the input pump-power islarge enough to allow the resonant wave(s) to be multi-longitudinal modeand/or linewidth-broadened. The OPO is provided with NLO crystals havinga broad phasematching bandwidth and a broadened gain-curve (e.g., by achirped ferroelectric poling, or with multi gain peaks by using multipoling crystal chips). The light source may optionally include otherelements, e.g., thick etalon for further gain shaping. If desired,additional broadening by longitudinal mode-scrambling may be carried out(e.g., by cavity-dither or etalon dither or diffraction grating ditheror crystal poling period dither). Additional intracavity NLO processes(e.g., SHG of the resonant wave or SFG with the resonant wave) mayoptionally result in efficient generation of different wavelengths. Assuch, a cw-OPO resonant on multi-mode (greater than 3 modes) isprovided. According to the present invention, any generation involvingthe resonant OPO-wave is very efficient due to the high intracavityresonant wave power. For example, for some OPOs the power-level of aresonant OPO wave may exceed 10 W, 100 W or even 1000 W. Beneficially,the light source of the present invention is capable of efficientlyproducing radiation in the VIS/IR wavelength ranges. Such a light systemis capable of generating light suitable for applications includingmicroscopy and biotech.

In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A light source comprising: a pump sourceconfigured to produce a continuous wave pump beam; and an OpticalParametric Oscillator (OPO) comprising an optical cavity containing anonlinear crystal configured to receive the continuous wave pump beamand produce a first output light beam and a second output light beam,the OPO having an OPO-oscillation threshold level, wherein the pumpsource is configured to produce the pump beam at a power exceeding about3 times the OPO-oscillation threshold level such that the OPO producesbroadband and/or multi-mode emission and the optical cavity isconfigured to resonate the first output light beam in amulti-longitudinal resonant mode.
 2. The light source of claim 1,wherein the pump source is configured to produce the pump beam at apower exceeding about 5 times the OPO-oscillation threshold level. 3.The light source of claim 1 further comprising a longitudinalmode-scrambling mechanism disposed on the resonant OPO-wave.
 4. Thelight source of claim 3, where the longitudinal mode-scramblingmechanism is a mechanical scrambling mechanism.
 5. The light source ofclaim 4, wherein the longitudinal mode-scrambling mechanism is selectedfrom one or more of a dithered optical resonator length, an intra-cavityetalon, a refractive grating and a non-linear crystal, wherein thelongitudinal mode-scrambling mechanism is configured to modulate aphase-matching condition and/or wavelength-dependent gain or losses ofthe OPO.
 6. The light source of claim 3, where the longitudinalmode-scrambling mechanism is selected from one or more of anintra-cavity electro-optic phase modulator, an intra-cavityelectro-optic etalon, or a non-linear crystal, wherein the longitudinalmode-scrambling mechanism is configured to electro-optically modulatethe nonlinear phase-matching condition and/or wavelength-dependent gainor losses of the OPO.
 7. The light-source of claim 1, further comprisinga second nonlinear crystal within an optical resonator of the OPO,wherein the second crystal is configured for second harmonic generation(SHG), sum frequency generation (SFG) or OPO processes.
 8. The lightsource of claim 7, wherein the second crystal is configured to enablethe light source to access additional wavelength ranges with broadbandand/or multimode emission having a bandwidth greater than 300 GHz. 9.The light source of claim 7, further comprising a second pump-sourceconfigured to pump the second nonlinear crystal.